1. Field of the Invention
The present invention relates to a method of performing communication in a wireless communication system, and more particularly, to a method of transmitting reference signals in a wireless communication system having multiple antennas.
2. Discussion of the Related Art
A 3rd Generation Partnership Project (3GPP) wireless communication system based on the Wideband Code Division Multiple Access (WCDMA) radio access technology has been widely developed. A High Speed Downlink Packet Access (HSDPA) that can be defined as a first evolution stage of the WCDMA provides the 3GPP with a radio access technology having high competitiveness in the mid-term future.
An example of the radio access technology for providing high competitiveness in the long-term future includes an Evolved Universal Mobile Telecommunications System (E-UMTS). The E-UMTS is an evolved version of the conventional WCDMA UMTS, and its basic standardization is in progress under the 3GPP. The E-UMTS is also referred to as a Long Term Evolution (LTE) system. For details of the technical specifications of the UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network.”
The E-UMTS includes a User Equipment (UE), a base station, and an Access Gateway (AG) which is located at an end of a network (E-UTRAN) and connected to an external network. Generally, the base station can simultaneously transmit multiple data streams for a broadcast service, a multicast service and/or a unicast service. In the LTE system, an orthogonal frequency divisional multiplexing (OFDM) scheme and a multiple-input multiple-output (MIMO) scheme are used to downlink-transmit various services.
The OFDM scheme represents a high-speed data downlink access system. The OFDM scheme has an advantageous of high spectral efficiency in that the whole spectrum as allocated can be used by all base stations. For OFDM modulation, a transmission band is divided into a plurality of orthogonal subcarriers in a frequency domain and a plurality of symbols in a time domain. Since the OFDM scheme divides the transmission band into a plurality of subcarriers, a bandwidth per subcarrier is reduced and a modulation time per carrier is increased. Since the plurality of subcarriers are transmitted in parallel, a transmission rate of digital data or symbols of a specific subcarrier is more lowered than that of a single carrier.
The MIMO scheme is used for a communication system with a plurality of transmitting and receiving antennas. The MIMO scheme can linearly increase channel capacity without additional increase of a frequency bandwidth as the number of transmitting and receiving antennas increases. Examples of the MIMO scheme include a spatial diversity scheme and a spatial multiplexing scheme, wherein the spatial diversity scheme can enhance transmission reliability using symbols which have passed through various channel paths, and the spatial multiplexing scheme is to increase a transmission rate by simultaneously transmitting respective data streams from the respective antennas using a plurality of transmitting antennas.
Also, the MIMO scheme can be divided into an open-loop MIMO scheme and a closed-loop MIMO scheme depending on whether a transmitter knows channel information. In the open-loop MIMO scheme, the transmitter does not know channel information. Examples of the open-loop MIMO scheme include PARC (per antenna rate control), PCBRC (per common basis rate control), BLAST (Bell Laboratories Layered Space Time), STTC (space time trellis code), random beamforming, etc. On the other hand, in the closed-loop MIMO scheme, the transmitter knows channel information. Throughput of the closed-loop MIMO scheme depends on how exactly the transmitter knows the channel information. Examples of the closed-loop MIMO scheme include PSRC (per stream rate control), TxAA (Transmit Antenna Array), etc.
The channel information means radio channel information (for example, attenuation, phase shift, or time delay, etc.) between a plurality of transmitting antennas and a plurality of receiving antennas. In the MIMO scheme, various stream paths exist in accordance with combination of a plurality of transmitting and receiving antennas. The MIMO scheme has fading characteristics that a channel status is irregularly changed in time/frequency domains depending on time due to multi-path time delay. Accordingly, a receiver obtains channel information through channel estimation. The channel estimation is to estimate channel information to recover a distorted transmission signal. For example, the channel estimation means that amplitude and reference phase of carriers are estimated. Namely, the channel estimation is to estimate frequency response of a radio interface or a radio channel.
An example of a channel estimation method includes a method of estimating a reference value based on reference signals (RS) of several base stations using a two-dimensional channel estimator. The reference signals (RS) mean symbols having no real data but having high output to assist carrier phase synchronization and acquisition of base station information. The transmitter and the receiver can perform channel estimation using such reference signals (RS). Channel estimation through the reference signals (RS) means that a channel is estimated through a symbol commonly known by the transmitter and the receiver and data are recovered using the estimated result. The reference signals (RS) are also referred to as pilots.
The MIMO scheme supports a time division duplexing (TDD) mode and a frequency division duplexing (FDD) mode. Since forward link transmission and reverse link transmission lie on the same frequency domain in the TDD mode, a forward link channel can be estimated from a reverse link channel by the reciprocity principle.
Although the wireless communication technology developed based on WCDMA has been evolved into LTE, request and expectation of users and providers have continued to increase. Also, since another wireless access technology is being continuously developed, new evolution of the wireless communication technology is required for competitiveness in the future. In this respect, reduction of cost per bit, increase of available service, use of adaptable frequency band, simple structure, open type interface, proper power consumption of user equipment, etc. are required.
In this respect, standardization of advanced technology of LTE is in progress under the 3rd Generation Partnership Project (3GPP). Herein, this technology will be referred to as “LTE-Advance” or “LTE-A.” One of important differences between the LTE system and the LTE-A system is the number of antennas for transmission. Currently, the LTE system aims to support a single antenna. On the other hand, the LTE-A system aims to support multiple antennas that reach maximum four antennas. Accordingly, the LTE-A system should support transmission of reference signals for maximum four antennas. Particularly, in the LTE-A system, a method for supporting multi-user MIMO (MU-MIMO) is discussed.